1. Field of the Invention
The present invention relates to a switching element and a display device comprising this switching element.
2. Description of the Related Art
Image display devices are now widely used as display sections of computers, televisions, and the like. Typical known examples of image display devices are cathode ray tube displays, liquid crystal displays, or organic electro luminescence displays.
In recent years, flat panel displays (FPD) such as liquid crystal displays and organic electro luminescence displays have been widely used in place of CRT displays, which were previously widely used. This is because the FPD is lighter and thinner and is thus portable and saves space.
As basic parts of the FPD, thin-film transistors (TFT) have been widely used as switching elements that accomplish active matrix driving. Furthermore, TFT substrates have been widely used
The TFT substrate includes a large number of fine TFTs arranged on a substrate. Typically, the TFTs arranged in a matrix each include a gate electrode, a source electrode, and a drain electrode. By supplying a gate signal and an address signal to the gate electrode and the source electrode, respectively, the TFTs are sequentially scanned to accurately represent two-dimensional information on an image.
The TFT substrate is manufactured by depositing and patterning a semiconductor film, an insulating film, or a conductor film on an insulating substrate using a vacuum apparatus such as a plasma induced chemical deposition apparatus, a sputtering apparatus, or a dry etching apparatus. The TFT substrate is expensive because it is manufactured through such a large number of complicated manufacturing steps. In particular, a large vacuum apparatus is required to produce large substrates. However, the above vacuum apparatuses are expensive, and in particular larger vacuum apparatuses are very expensive. This further increases manufacturing costs.
In this case, the large substrate refers mainly to a substrate of 20 inches or more diagonal. A display including such a large substrate is called a large display. In recent years, larger FPDs have been desired to be developed in order to provide large (20 inches or more diagonal) and thin televisions.
Instead of TFT substrates which are expensive and are difficult to make larger, Buzaku et. al proposes in Japanese Patent Laid-Open No. 9-120270 an address device utilizing a discharge plasma switch.
With reference to FIG. 20, a brief description will be given of the address device 700 disclosed in Japanese Patent Laid-Open No. 9-120270. FIG. 20 is a perspective view schematically showing the address device 700. In FIG. 20, reference numeral 701 schematically shows that discharge is occurring. The address device 700 has a first substrate and a second substrate (neither of them are shown) and a dielectric layer 44 provided between the first and second substrates.
The second substrate has a plurality of parallel stripes-like grooves. Each of the grooves and the dielectric layer 44 define a sealed plasma channel (discharge channel). A gas that can be ionized by discharge is sealed in the plasma channel. A pair of discharging electrodes 41 and 42 is provided at the bottom of the groove. One of the electrodes constituting the pair is used as an anode (A), while the other is used as a cathode (K). A voltage is applied to the gas sealed in the plasma channel to ionize the gas to generate discharge plasma. The term “activate” or “activation” may be used to express the operation of generating plasma discharge to ionize the gas present in the plasma channel.
On the other hand, a plurality of parallel stripes-like transparent electrodes 49 are provided on a surface of the first substrate which is opposite to the dielectric layer 44. The stripes-like transparent electrodes 49 are arranged to cross the stripes-like grooves (i.e. the plasma channels) formed in the second substrate. The crossing portions define individual address areas arranged in a matrix.
In this address device, for example, the plasma channels constitute row scanning units, whereas the transparent electrodes 49 constitute column driving units. Selective plasma discharge is used to sequentially activate the plasma channels to execute line sequential scanning for each raw. In synchronization with this, a driving signal is applied to each of the transparent electrodes 49, constituting the column driving units. When plasma discharge is used to activate the plasma channel, an anode potential level is almost uniformly established in the plasma channel. Then, one end of the dielectric layer 44 in each address area is connected to the anode electrode via a surface 44′ of the dielectric layer. In this manner, the plasma channel constitutes what is called a plasma switch. When the switch is turned on, a driving signal is synchronously applied to the other end of the dielectric layer 44 in each address area. Charges equivalent to a difference in potential are accumulated in the dielectric layer 44. The applied driving signal is held even after the plasma switch has been turned off. That is, what is called “sample and hold” is carried out.
Compared to TFT substrates, it is easy to increase the size of the address device 700, which utilizes the discharge plasma switches as described above, and the manufacturing costs of this device are low. However, the address device 700 creates the problems described below.
With the address device 700, a switching operation is performed utilizing the characteristic that the activation of each plasma channel based on plasma discharge causes the anode potential level to be almost uniformly established in the plasma channel, as described above. However, actually, a sheath voltage attributed to plasma is present, so that the potential on the surface 44′ of the dielectric layer is not the same as the anode potential.
Thus, a voltage equivalent to the difference between the potential on the surface 44′ of the dielectric layer and the anode potential must be applied, as a bias, to the voltage of the driving signal applied to the transparent electrodes 49. This bias voltage varies depending on the shapes of the discharge electrodes 41 and 42 and plasma channels. Accordingly, the bias voltage must be set for each address device. Furthermore, if the shapes of the discharge electrodes 41 and 42 and plasma channels vary significantly with areas within the single address device, the bias voltage must be varied depending on the areas within the single address device.
In the address device 700, data is written and accumulated by accumulating charges on the surface 44′ of the dielectric layer. However, plasma generated in each plasma channel is nonuniformly distributed around the discharging electrodes 41 and 42. Accordingly, charge accumulated on the surface 44′ of the dielectric layer is also nonuniformly distributed. Thus, data (that is, charge) accumulated in each address area are nonuniform within the address area. Consequently, if the address device 700 is used as a display device, display will be nonuniform in a pixel area corresponding to each address area.
Moreover, in the address device 700, data are written and accumulated using charged particles of plasma generated in each plasma channel. However, depending on the type or pressure of a discharge gas sealed in the plasma channel, charged particles may remain in the plasma channel for a relatively long time even after the anode potential has become 0 V. The remaining charged particles may affect the write or accumulation of data.
The time for which charged particles remain depends on the type of the discharge gas and the sealing pressure. Accordingly, the type of the discharge gas and the sealing pressure must be optimized in order to increase switching speed. If the optimization is not carried out, the achieved switching speed will be low. Even if such a discharge gas as achieves a high switching speed is used, problems such as an increased discharge voltage may occur.
The present invention is provided in view of these problems. It is an object of the present invention to provide a switching element that can be easily manufactured and a display device comprising this switching element.